This application is based on an application No. H11-343528 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a video recording device for recording a video signal which includes character information such as closed caption data, and in particular relates to a technique of recording character information together with video information.
2. Prior Art
A video recording device equipped with a time base corrector (hereinafter, xe2x80x9cTBCxe2x80x9d) is conventionally used to receive analog video signals outputted from a TV broadcaster or a video recorder on a frame-by-frame basis, convert the received analog video signals to digital video signals, and record them onto a recording medium such as a DVD.
In general, when converting an analog video signal to a digital video signal and recording it, the receive frequency of the video signal and the clock frequency which defines the timing of recording the video signal onto a recording medium (hereinafter, xe2x80x9creference frequencyxe2x80x9d) do not synchronize precisely. This being so, when video signals are sequentially being written into an internal memory of the video recording device in accordance with the receive frequency and then being read from the internal memory and recorded onto the recording medium in accordance with the reference frequency, the amount of data flowing into the memory and the amount of data flowing from the memory become unbalanced, which often causes the memory to underflow or overflow.
To avoid this and enable seamless recording of data, the TBC operates to monitor the occurrence of underflow and overflow in the memory, and smooth out discrepancies between the video signal recording speed defined by the reference frequency which is fixed and the video signal input speed determined by the receive frequency which could vary, by repeating or skipping an output of a video signal from the internal memory in units of video frames. In so doing, the balance between the amount of incoming data and the amount of outgoing data in the memory is maintained.
To be more specific, when the receive frequency is higher than the reference frequency, the amount of incoming data exceeds the amount of outgoing data per unit time. In this case, the TBC skips a one-frame video signal and outputs the next one-frame video signal from the memory. On the other hand, when the receive frequency is lower than the reference frequency, the amount of incoming data falls short of the amount of outgoing data per unit time. In this case, the TBC outputs a one-frame video signal which was once outputted, again.
FIG. 1 shows how the TBC repeats or skips an output of a one-frame video signal.
In the figure, each large box represents a one-frame video signal which includes video information for video display and character information for character display.
The video information is designated by an alphabet that identifies the video signal, to indicate the flow of the video signal being written into or read from the memory.
The character information (explained later) is designated by alphabets that show characters contained in two types of character information included in the video signal. The uppercase alphabetic characters and the lowercase alphabetic characters respectively correspond to the two types of character information.
In each video signal shown in the middle of the figure, the alphabet given above the forward slash mark xe2x80x9c/xe2x80x9d indicates data which is read from the memory in the case where the receive frequency is higher than the reference frequency (t1 less than t0, t0=1/(reference frequency)), whereas the alphabet given under xe2x80x9c/xe2x80x9d indicates data which is read from the memory in the case where the receive frequency is lower than the reference frequency (t2 greater than t0).
In the figure, the direction from left to right represents time, with the reference line of each frame with respect to time is indicated by the left side line.
The operation of the TBC is explained below, with reference to this figure.
Unless the video signal receive frequency keeps perfect synchronization with the reference frequency, the amount of data flowing into the memory and the amount of data flowing from the memory will eventually become unbalanced, thereby causing an overflow or an underflow in the memory.
To prevent a memory overflow (in the case of t1 less than t0), the TBC skips the reading of the one-frame video signal xe2x80x9cDxe2x80x9d (see X1 in FIG. 1), and instead reads the next one-frame video signal xe2x80x9cExe2x80x9d from the memory. To prevent a memory underflow (in the case of t2 greater than t0), the TBC reads the one-frame video signal xe2x80x9cAxe2x80x9d twice from the memory (see X2 in FIG. 1).
In recent years, a caption display technique called closed captioning has been developed to display a caption of actors"" lines, narrations, and sound effects so that the hearing impaired can enjoy broadcast programs and videos. In the closed caption technique, character information for caption display is multiplexed on a video signal so as to display a caption in sync with a video.
Also, similar caption display techniques have been developed that display information other than closed captions, such as weather forecast information, time information, and stock price information, by multiplexing these information on a video signal. Nowadays these techniques are put into actual use as well as the closed captioning.
Consequently, a video signal which usually contains only video information comes to contain character information such as closed caption data, when these caption display techniques are employed.
Such character information is carried on line 21 in both fields of the video signal, i.e. the vertical interval which is not used for picture display.
To display such character information on a screen, a decoder for decoding the character information multiplexed on the video signal is needed. A viewer can decide whether to display the character information or not.
However, the aforementioned caption display techniques have the following problem.
Video information corresponding to one frame is visual information of approximately 29.97 msec. A dropout or overlap of such one-frame video information is unlikely to cause visual disturbances. However, the problem arises when the TBC skips or repeats recording of character information corresponding to one frame, in the same way as video information. Character information is made up of character codes such as ASCII (American Standard Code for Information Interchange) codes and special character codes. Suppose character information representative of the characters xe2x80x9cA, Bxe2x80x9d, xe2x80x9cC, Dxe2x80x9d, xe2x80x9cE, Fxe2x80x9d, xe2x80x9cG, Hxe2x80x9d, xe2x80x9cI, Jxe2x80x9d, . . . is sequentially inputted into the memory as closed-caption data, as shown in FIG. 1. Here, if the receive frequency is higher than the reference frequency, the character information is recorded in the order of xe2x80x9cA, Bxe2x80x9d, xe2x80x9cC, Dxe2x80x9d, xe2x80x9cE, Fxe2x80x9d, xe2x80x9cI, Jxe2x80x9d, . . . , where the character information xe2x80x9cG, Hxe2x80x9d is skipped. If, on the other hand, the receive frequency is lower than the reference frequency, the character information is recorded in the order of xe2x80x9cA, Bxe2x80x9d, xe2x80x9cA, Bxe2x80x9d, xe2x80x9cC, Dxe2x80x9d, xe2x80x9cE, Fxe2x80x9d, . . . , where the character information xe2x80x9cA, Bxe2x80x9d is recorded twice. Thus, when the TBC is used to record character information multiplexed on video signals, dropouts or overlaps of characters are likely to occur. Character information recorded in such a way will end up being unintelligible to a viewer.
In view of the above problem, the first object of the invention is to provide a video recording device that, when recording a video signal containing character information using a TBC, can record the character information as faithfully as possible.
The second object of the invention is to provide a video recording method that realizes the faithful recording of the character information.
The third object of the invention is to provide a video rate adjusting device that outputs a video signal for enabling the faithful recording of the character information, the video rate adjusting device having the construction of the video recording device without a recording medium.
The first object can be fulfilled by a video recording device for recording a plurality of sets of video information that include a plurality of sets of character information, into a recording medium, including: a receiving unit operable to sequentially receive the plurality of sets of video information at a receive frequency which may vary; an extracting unit operable to sequentially extract the plurality of sets of character information from the received plurality of sets of video information; a video outputting unit operable to output the received plurality of sets of video information one at a time at a fixed output frequency which does not depend on the receive frequency, wherein when there is a frequency discrepancy between the output frequency and the receive frequency, the video outputting unit (1) skips a set of video information or (2) outputs a set of video information which was once outputted, again; a character outputting unit operable to output the extracted plurality of sets of character information one at a time at the output frequency, in the extracted order; a recording unit operable to record the outputted plurality of sets of video information and the outputted plurality of sets of character information in combination, into the recording medium in the outputted order; and an abandoning unit operable to, whenever a predetermined condition is met, (a) judge whether a value included in a set of character information extracted by the extracting unit is null, (b) abandon the set of character information whose value is judged as being null, and (c) instruct the character outputting unit to skip the abandoned set of character information and instead output a set of character information immediately following the abandoned set of character information.
With this construction, when the predetermined condition is present which indicates a danger of a buffering overflow, the set of character information which is judged as being null is abandoned and instead the next set of character information is outputted from the character outputting unit. As a result, a buffering overflow is avoided.
Here, the video recording device may further include: a buffer operable to temporarily store data, in which the plurality of sets of character information extracted by the extracting unit are sequentially inputted; and a data amount detecting unit operable to detect an amount of data currently stored in the buffer, wherein the predetermined condition is that the amount of data currently stored in the buffer is no smaller than a first threshold value, the character outputting unit retrieves a set of character information which has been stored for a longest time in the buffer, and outputs the retrieved set of character information, and when the predetermined condition is met, the retrieved set of character information is subjected to the judgement by the abandoning unit, before being outputted by the character outputting unit.
With this construction, when the amount of data stored in the buffer is equal to or larger than the first threshold value, the set of character information which is judged as being null is abandoned, thereby keeping the buffer from overflowing.
Here, the video recording device may further include a null information generating unit operable to generate a set of null information whose value is null, wherein if the amount of data currently stored in the buffer is below a second threshold value, the character outputting unit outputs the set of null information generated by the null information generating unit, as a set of character information.
With this construction, when the amount of data stored in the buffer is smaller than the second threshold value, the set of information that is null is generated and outputted from the character outputting unit, which enables seamless output of the plurality of sets of character information.
Here, the recording medium may be a DVD, wherein at least one of the plurality of sets of video information received by the receiving unit includes no set of character information, each of the received plurality of sets of video information includes inclusion information showing whether the set of video information includes a set of character information or not, and the recording unit (a) attaches a flag based on DVD recording standards which shows the inclusion of a set of character information, to each of the plurality of sets of video information outputted from the video outputting unit, regardless of whether the inclusion information of each of the plurality of sets of video information shows the inclusion or non-inclusion of a set of character information, and (b) records the combination of the outputted plurality of sets of video information and the outputted plurality of sets of character information, into the recording medium as a single VOB (Video Object).
With this construction, the flag showing the presence of a set of character information is attached to each of the plurality of sets of video information which are recorded as a single VOB, irrespective of whether each of the plurality of sets of video information actually includes a set of character information or not. In so doing, the DVD recording standards are complied with.
Here, the flag may be line 21 switch based on the DVD recording standards.
With this construction, line 21 switch is held ON to show the presence of a set of character information throughout the plurality of sets of video information which are recorded as a single VOB, irrespective of whether each of the plurality of sets of video information actually includes a set of character information or not. In so doing, the DVD recording standards are complied with.
Here, a value included in at least one of the plurality of sets of character information extracted by the extracting unit may be a character code used for closed captioning.
With this construction, when the predetermined condition is present which indicates a danger of a buffering overflow, closed caption data which is null is abandoned, with it being possible to avoid a buffering overflow without obstructing a faithful character display in reproduction.
Here, the recording medium may be a DVD, wherein at least one of the plurality of sets of video information received by the receiving unit includes no set of character information, each of the received plurality of sets of video information includes inclusion information showing whether the set of video information includes a set of character information or not, and the recording unit (a) attaches a flag based on DVD recording standards which shows the inclusion of a set of character information, to each outputted set of video information whose inclusion information shows the inclusion of a set of character information, (b) attaches a flag based on the DVD recording standards which shows the non-inclusion of a set of character information, to each outputted set of video information whose inclusion information shows the non-inclusion of a set of character information, and (c) records the combination of the outputted plurality of sets of video information and the outputted plurality of sets of character information into the recording medium as separate VOBs, the VOBs being separated from each other whenever the flag changes.
With this construction, when the state of the flag that shows the presence or absence of a set of character information changes from one video information string to the next video information string, the two video information strings are recorded as separate VOBs, with it being possible to comply with the DVD recording standards.
Here, the flag may be line 21 switch based on the DVD recording standards.
With this construction, when line 21 switch changes from one video information string to the next video information string, the two video information strings are recorded as separate VOBs, with it being possible to comply with the DVD recording standards.
Here, the recording medium may be a DVD, wherein at least one of the plurality of sets of video information received by the receiving unit includes no set of character information, each of the received plurality of sets of video information includes inclusion information showing whether the set of video information includes a set of character information or not, and the recording unit (a) attaches, when inclusion information of a set of video information outputted first among the plurality of sets of video information shows the inclusion of a set of character information, a flag based on DVD recording standards which shows the inclusion of a set of character information, to each of the plurality of sets of video information outputted from the video outputting unit, regardless of whether inclusion information of each of the plurality of sets of video information other than the first outputted set of video information shows the inclusion or non-inclusion of a set of character information, (b) attaches, when the inclusion information of the first outputted set of video information shows the non-inclusion of a set of character information, a flag based on the DVD recording standards which shows the non-inclusion of a set of character information, to each of the outputted plurality of sets of video information, regardless of whether the inclusion information of each of the plurality of sets of video information other than the first outputted set of video information shows the inclusion or non-inclusion of a set of character information, and (c) records the combination of the outputted plurality of sets of video information and the outputted plurality of sets of character information, into the recording medium as a single VOB.
With this construction, the inclusion information of the set of video information at the beginning of the plurality of sets of video information determines the state of the flag throughout the plurality of sets of video information which are recorded as a single VOB, so that the DVD recording standards are complied with.
Here, the flag may be line 21 switch based on the DVD recording standards.
With this construction, the inclusion information of the set of video information at the beginning of the plurality of sets of video information determines the state of line 21 switch throughout the plurality of sets of video information which are recorded as a single VOB, so that the DVD recording standards are complied with.
Here, the plurality of sets of character information extracted by the extracting unit may form packet data that contains a packet header, wherein the video recording device further includes a judging unit operable to judge whether a value included in the set of character information retrieved from the buffer shows the start of the packet header, wherein if the amount of data currently stored in the buffer exceeds a third threshold value which is larger than the first threshold value, and if the judging unit judges that the value in the retrieved set of character information shows the start of the packet header, the abandoning unit (a) abandons a predetermined number of consecutive sets of character information starting from the set of character information whose value is judged as showing the start of the packet header, and (b) instructs the character outputting unit to skip the abandoned sets of character information, and instead output a set of character information immediately following the abandoned sets of character information.
With this construction, when the amount of data stored in the buffer exceeds the third threshold value, the predetermined number of sets of character information starting from the set of character information which corresponds to the start of the packet are abandoned, with it being possible to avoid an overflow of the buffer.
Here, each of the plurality of sets of character information extracted by the extracting unit may include error information which shows whether an error is present in a value included in the set of character information or not, wherein the video recording device further includes an error judging unit operable to refer to error information in the set of character information extracted by the extracting unit, and judge whether the error information shows the presence or absence of an error, wherein when the error information shows the presence of an error, the character outputting unit changes the error information to show the absence of an error, and outputs the set of character information
With this construction, even when the error information in the set of character information shows the presence of an error, the error information is changed to show the absence of an error before the set of character information is outputted from the character outputting unit, so that the DVD recording standards are complied with.
Here, when the error information shows the presence of an error, the character outputting unit may change the error information to show the absence of an error, further change a value included in the set of character information to a predetermined value, and output the set of character information.
With this construction, even when the error information in the set of character information shows the presence of an error, the value in the set of character information is changed to the predetermined value and the error information is changed to show the absence of an error, before the set of character information is outputted from the character outputting unit. In so doing, the DVD recording standards are complied with.
Here, the predetermined value may be a character code which represents a box filled in with black.
With this construction, even when the error information in the set of character information shows the presence of an error, the value in the set of character information is changed to a character code representative of a box filled in with black, and the error information is changed to show the absence of an error, before the set of character information is outputted from the character outputting unit. In so doing, the DVD recording standards are complied with.
Here, the error information may be a parity bit, wherein the character outputting unit changes a parity bit in the set of character information whose value is judged as having an error, to show a correct parity, and outputs the set of character information.
With this construction, even when the parity bit in the set of character information shows the presence of an error, the parity bit is corrected before the set of character information is outputted from the character outputting unit.
Here, when the error information shows the presence of an error, the character outputting unit may change the error information to show the absence of an error, further change a value included in the set of character information to a null value, and output the set of character information.
With this construction, even when the error information in the set of character information shows the presence of an error, the value in the set of character information is changed to a null value and the error information is changed to show the absence of an error, before the set of character information is outputted from the character outputting unit. In so doing, the DVD recording standards are complied with.
Here, the video recording device may further include: a word storing unit storing a plurality of different words; and a character acquiring unit operable to (a) obtain a character string which seems to form a word, based on characters represented by values included in sets of character information preceding and following the set of character information whose value is judged as having an error, (b) search the word storing unit for a word corresponding to the obtained character string, to find a character which seems to be a correct character for a character represented by the value in the set of character information, and (c) generate a character code representative of the found character, wherein the character outputting unit changes the value in the set of character information to the generated character code, and outputs the set of character information.
With this construction, even when the error information in the set of character information shows the presence of an error, the value in the set of character information is replaced with a character code representative of a character which seems to be correct, and the error information is changed to show the absence of an error, before the set of character information is outputted from the character outputting unit. In so doing, the DVD recording standards are complied with.
Here, the character acquiring unit may obtain the character string, on an assumption that the character string ends right before a character other than an alphabet.
With this construction, the word corresponding to the character string which contains the wrong character is extracted, so as to find the correct character.
The second object can be fulfilled by a video recording method for recording a plurality of sets of video information that include a plurality of sets of character information, into a recording medium, including: a receiving step for sequentially receiving the plurality of sets of video information at a receive frequency which may vary; an extracting step for sequentially extracting the plurality of sets of character information from the received plurality of sets of video information; a video outputting step for outputting the received plurality of sets of video information one at a time at a fixed output frequency which does not depend on the receive frequency, wherein when there is a frequency discrepancy between the output frequency and the receive frequency, the video outputting step (1) skips a set of video information or (2) outputs a set of video information which was once outputted, again; a character outputting step for outputting the extracted plurality of sets of character information one at a time at the output frequency, in the extracted order; a recording step for recording the outputted plurality of sets of video information and the outputted plurality of sets of character information in combination, into the recording medium in the outputted order; and an abandoning step for, whenever a predetermined condition is met, (a) judging whether a value included in a set of character information extracted by the extracting step is null, (b) abandoning the set of character information whose value is judged as being null, and (c) instructing the character outputting step to skip the abandoned set of character information and instead output a set of character information immediately following the abandoned set of character information.
With this construction, when the predetermined condition is present which indicates a danger of a buffering overflow, the set of character information which is judged as being null is abandoned and instead the next set of character information is outputted from the character outputting unit, so that a buffering overflow is avoided.
The third object can be fulfilled by a video rate adjusting device, including: an input accepting unit operable to sequentially accept a plurality of videos and a plurality of sets of character information; a video outputting unit operable to skip part of the accepted plurality of videos, and output the remaining videos at a predetermined speed in the accepted order; a character judging unit operable to judge, for each of the accepted plurality of sets of character information, whether a value included in the set of character information is null; and a character outputting unit operable to output, in a one-to-one correspondence with the videos outputted by the video outputting unit, sets of character information other than the sets of character information whose values are judged as being null, at the predetermined speed in the accepted order.